Self-balancing bridge arrangement



Feb. 22, 1949 A. D. BLUMLEHN ET AL 2,452,599

SELF-BALANCING BRIDGE ARRANGEMENT Filed Sept. 11, 1943 ATTORNEY UNITED STATES PATENT OFFICE 2.462.599 SELF-BALANCING BRIDGE ARRANGEMENT Alan Dower Blumlein, deceased, late of Ealing, London, England, by Doreen Blumlein, executrix, Lescudjack, Penzance, Cornwall, England, William Horace Connell, Hillingdon, and Dennis Godson Holloway, Maidenhead, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application September 11, 1943, Serial No. 501,952 In Great Britain November 21, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires November 21, 1961 Claims. (01. 323-75) 1 This invention relates to self-balancing bridge In order that the nature of the invention may arrangements. be more fully understood and readily carried into Bridge arrangements have been proposed in efifect it will now be more fully described by way which in operation an out-of-balance voltage or of example as applied to apparatus for indicating current is developed across the bridge, and this 5 the height of an aircraft, with reference to the voltage or current is caused to adjust one of the accompanying drawings, in which:

components of the bridge so as always to tend Figure 1 shows a general schematic circuit arto maintain the bridge in balance. rangement of the invention,

If the component which is adjusted is of the Figures 2 and 3 show an adjustable capacity kind in which adjustment is obtained, for exfor the circuit arrangement of Figure 1 in planample, by relative movement between two elecand in side elevation respectively, and trodes, and the relative position of these elec- Figure 4 shows the variation of capacity with .trodes is used to indicate the value of the comthe angular position of the moving electrode of ponent being measured, the scale law of the inthe variable capacity shown in Figures 2 and 3.

dicator may be unsatisfactory if the character- Referring now to Figure 1, the self-balancing is'tic, i. e. the relation between the out-of-balance bridge arrangement will be seen to be of the kind voltage or current and the admittance of the adforming the subject matter of- British patent ap- .justed component is non-linear, since for a given plication serial No. 446,972, filed June 13, 1942,

change of admittance the relative movement of in which a capacity I between two electrodes (not the electrodes will not be the same at all points go shown), which is a function of the height of an in the range of movement. aircraft above a conducting surface, such as the It is thereforedesirable to control the shape sea, is measured by means of a bridge having of the characteristic of the adjusted component tightly coupled inductive ratio arms 2 and 3 and and it is the object of the present invention to an adjustable balancing capacity. As shown in provide means for effectively controlling said the figure, the generator 4 supplies the alternatcharacteristic. ing bridge current on the One hand through the According to the present invention there is procapacity I to be measured and the inductive ravided a self-balancing bridge arrangement in tio arm 2, and on the other hand to the balancing which out-of-balance voltages or currents decapacity comprising the moving electrode 5 and veloped in said bridge in operation are caused tothe two fixed electrodes 6, 1 connected respecadjust a balancing component so as to tend to tively to the inductive ratio arm 3 and to a tapmaintain said bridge balanced, said balancing ping point intermediate the ends of the inductive component having a non-linear characteristic inratio arm 2. Out-of-balance voltages are picked eluding a portion of low slope and in which a up in the coupling coil 8 and after amplification further component connected in said bridge so as by the amplifier indicated schematically by the effectively to oppose said balancing component is rectangle 9 control the angular position of the arranged to be adjusted by said out-of-balance electrode 5 by means of currents passing through voltages or currents so that the admittances of the operating coil of a movement, such as a weak said further and said balancing components are current meter movement, attached to electrode changed in opposite senses whereby the charac- 5. The bridge arrangement thus differs from teristic of said balancing component is effectively that forming the subject matter of the aforesaid modified so as to increase the slope of said porapplication in that the automatically adjusted tion of low slope. bridge component is in the form effectively of two Preferably, said balancing component and said condensers instead of a single condenser, and, as further component are constituted by the cawill now be explained, the introduction of the pacities between each of the fixed electrodes and second balancing capacity and the adjustment the moving electrode respectively of a diiferential of its admittances in the opposite sense to the condenser, said moving electrode being arranged first capacity give important advantages. to be adjusted in position by said out-oflbalance As the variation of capacity I with the height current or voltages. of the aircraft is found to be substantially exdistance between the electrodes and the ordinates are proportional to the capacity between the electrodes and therefore proportional to the admittances of said capacity at the bridge frequency. It will be seen that as the distance between the electrodes increases, the capacity decreases substantially exponentially, so that the relation between the height of the aircraft and the distance between theelectrodes can be made substantially linear, thus giving a convenient scale law to the indicator.

A single balancing capacity having a characteristic of this kind has, however, been found to be inconvenient in practice. It will be appreciated that part of the capacity [between the electrodes attached to the aircraft does not vary with height and in order to increase sensitivity it has been found desirable to balance out this constant part of the capacity by means of a subsidiary capacity not controlled by out-of-balance voltages, but adjusted manually when the aircraft is at a considerable height above the earth. Thus, when this subsidiary capacity has been adjusted in these circumstances, the automatically adjusted condenser will be at a very low capacity setting with its electrodes widely separated. In these circumstances, very small changes of the capacity I will cause relatively large movements of the moving electrode 5, since-the condenser is then operating over the right-hand portion of the characteristic shown in Figure 4 where a large movement of the moving electrode is required to effect a small change of capacity or admittance.

This condition is undesirable because small changes in capacity I are liable to occur due to the stressing of the aircraft structure in flight, so that even when the aircraft is flying at a constant height the moving electrode may oscillate with a relatively large amplitude and may even freeze over at one limit of its travel. Further, as the movement of the moving electrode may also be markedly asymmetrical for equal but opposite small changes of capacity, it becomes difficult to effect the desired manual adjustmentof the subsidiary balancing capacity.

It is found that these difilculties can be overcome by introducing a further adjustable bridge component in such a manner that the out-ofbalance voltages or currents adjust the two bridge components as to cause their admittances to change in opposite senses. Thus. referring again to Figure 4. the dotted line B shows the variation of the capacity or admittance between the electrodes 5 and I as the di tance between these electrodes is decreased by the movement of the electrode 5 away from the electrode 6.

Due to the connection of the electrode 1 to the opposi e bridge arm 2 from that to which the electrode 6 is connected the two bridge components comprising the c ndensers 5, I and 5, 6 respectivey are effectively in opposition so far as the bridge is concerned. Further, as the current flowing through the admittance between the electrodes 5, 1 only flows through a portion of the arm 2, its effect upon the bridge balance is reduced and it becomes equivalent to a smaller admittance connected to the outer end of the arm 2. .The admittance between electrodes 5 and 1 therefore increases negatively according to an exporise to hunting.

'nential law as the admittance between electrodes 5 and 6 decreases although its maximum value is considerably less than the maximum value of the admittance between the'electrodes 5 and 8. Thus. the resultant characteristic can berep'resented by the full line C which is the resultant of curves A .and B, and it will be observed that, instead of the slope of this characteristic decreasing continuously towards a low slope at one limit as in the case of curve A, the slope of the resultant curve C continuously decreases to a given value greater than the lowest slope of curve A and thereafter increases again. Thus, the oscillations of the moving electrode above referred to are greatly.

reduced due to the greater slope of the characteristic C in the neighbourhood of the zero effective admittance or capacity value and also the movements of the pointer are caused to be more symmetrical.

The differential condenser 5, 6, I shown in the circuit arrangement of Figure 1 may conveniently take the form shown in Figures 2 and 3, which is a modification of the variable condenser described in the specification of the aforesaid patent application. This variable condenser com prises a weak current meter movement shown diagrammatically at H and having an insulating pillar l2 fixed to the moving element of the meter H and carrying a light conducting pointer IS. The pointer l3 projects through the radial slot M in the earthed conducting structure l5 comprising two parallel conducting metal plates of the form shown in plan in Figure 2 joined together on their corresponding straight and inner semi-circular edges by means of a conducting metal strip containing the radial slot ll. Two electrodes IS, IT, each in the form of a small slotted rectangular block, are supported in an insulated manner within the structure IS, the slot in each electrode being so shaped and positioned that the moving pointer l3 can enter it without electrical contact with the electrode. The pro!- vision of these slots avoids the possibility of metallic contact between the moving electrode lland either of the fixed electrodes H5 or i! which might disturb the bridge arrangement and give This variable condenser will be seen to difier from that shown in the specification of the aforesaid application chiefly due to the fact that two fixed electrodes are provided instead of one and, as explained in the aforementioned specification, the shape of the characteristic may be modified by varying the spacing between the members of the earthed structure It, as, for example, by the provision of slots to permit suitable distortion of these plates.

Although the capacities between the movin electrode l3 and each of the fixed electrodes [6,

l1. respectively, are similar, they can be made] efiectively dissimilar in order to give characteristics comparable to A and B as shown in Figure 4 by feeding bridge voltages of difierent amplitudes to the electrodes 6, l, or alternatively, feeding the bridge currents to the moving electrode 5 and passing the currents received'by the electrodes 8,

"I, through different numbers of turns of the ratio arms of the bridge. Thus, if an adjustable condenser of the kind shown in Figures 2 and 3 is used in the circuit arrangement of Figure 1, the fixed electrode I may be connected to a tapping in the inductive ratio arm 2, such that the bridge current flowing from the electrode! only passes through 6 as many turns on the ratio arm 2 as the number of turns in the ratio arm 3 through which flows the bridge current from electrode 6.

Alternatively, the effectiveness of the capacity between the electrode and 6 or 1 may be controlled My introducing a capacity potentiometer between the electrode and the bridge so that only a fraction of the current reaching the electrode 6 or I flows into the appropriate ratio arm of the bridge. Further, the bridge voltage applied to the moving electrode 5 need not be the same as that applied to the capacity I to be measured and may be reduced to any desired extent, for example, by means of a step-down transformer or a capacity potentiometer.

If desired, the electrodes l6, I! may be made different in size so as to give the desired different variations of capacity or admittance without requiring the application of different bridge voltages or the use of different tappings on the ratio arms of the bridge.

It will be understood that although in the arrangement above referred to the two adjustable components of the bridge have taken the form of a single differential condenser, separate components may be used provided that they are arranged to be controlled by the out-of-balance voltages or currents so as to adjust the admittances of the separate components in opposite senses. Further, if desired, one or more of the adjustable components may be variable inductances or resistances, the nature of the element being chosen to be appropriate to the nature of the components to be measured.

It will be appreciated that, as is usual with alternating current bridges, auto-transformer couplings may be replaced by transformer couplings and the source of bridge current and the source of out-of-balance voltage may be interchanged. Although the invention has been described with reference to a self-balancing bridge arrangement embodied in a height indicator, it will be appreciated that it is generally applicable to self-adjusting bridges for the measurement and indication of any electrical impedances.

What we claim is:

1. An electric bridge including a fixed ratio arm consisting of'two elements and a variable ratio arm consisting of two variable elements, said fixed ratio arm being connected to said variable ratio arm to form said electric bridge, and a variable element connected to said fixed ratio arm at a point intermediate its terminals and to said variable ratio arm and being variable in synchronism with one of said variable elements to alter its rate of variation.

2. An electric bridge including a fixed ratio arm consisting of two inductors and a variable ratio arm consisting of two variable condensers, said fixed ratio arm being connected to said variableratio arm to form said electric bridge, and a, third variable condenser connected to said fixed ratio arm at a point providing a difi'erent fixed ratio and to said variable ratio arm.

3. An electric bridge including a fixed ratio arm consisting of two inductors and a variable ratio arm consisting of two variable condensers, said fixed ratio arm being connected tosaid variable ratio arm to form said electric bridge, and a third variable condenser connected to said fixed ratio arm at a point providing a difierent fixed ratio and to said variable ratio arm, said third variable condenser being variable in synchronism with one of the variable condensers of said variable ratio arm to alter its rate of variation.

4. A self-balancing bridge arrangement in which out-of-balance voltages or currents developed in said bridge in operation are caused to adjust a balancing component so as to tend to maintain said bridge balanced, including in combination a pair of fixed arms, a first adjustable arm having a non-linear characteristic including a portion of low slope and a second adjustable arm effectively connected to said first adjustable arm and to a point on said fixed arms and connected to oppose said balancing component and means operated in response to said out-of-balance voltages or currents, and effectively coupled to said adjustable arms so that the admittances of said first and second adjustable arms are changed in opposite senses and at different rates for equal changes in said adjustable arms whereby the characteristic of said balancing component is eifectively modified so as to increase the slope of said portion of low slope.

5. A self-balancing bridge arrangement according to claim 4, in which said characteristic is substantially exponential with said portion of lower slope at one limit thereof, the first and second adjustable arms being relatively disposed and connected so that as the admittance of one of said adjustable arms decreases exponentially the admittance of the other of said adjustable arms increases exponentially and vice versa.

- DOREEN BLUMLEIN, Ezecutria: of Alan Dower Bmmlein, Deceased.

WILLIAM HORACE CONNEIL. DENNIS GODSON HOLLOWAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Alternating Current Bridge Methods, by Hague, 4th ed., 1938, D les 340-344. 

